Our professional Navy was drawn from a perilously small area, and there were 200,000 “Dutchmen and Dagos” among the crews of our mercantile marine.

The expression also crops up in A Master of Fortune (1901), a Captain Kettle novel by Cutcliffe Hyne:

“When the time comes,” said the little sailor grimly, “we shall be ready for them, and if they interfere with me, I shall make the Congo Free State people sit up. But in the mean while they are not here, and I don’t see that they need be expected. They can trace us up the Congo from Leopoldville, if you like, by the villages we stopped at–one, we’ll say, every two hundred miles–but then we find this new river, and where are we? The river’s not charted; it’s not known to any of the Free State people, or I, being in their steamboat service, would have been told of it; and the entrance is so well masked at its Congo end by islands, that no one would guess it was there. The Congo’s twenty miles wide where our river comes in, and very shallow, and the steamer-channel’s right at the further bank. If they’d another Englishman in their service up here, I’d not say; but don’t you tell me that the half-baked Dutchmen and Dagos who skipper their launches would risk hunting out a new channel, and blunder on it that way.”

I assume that “Dutchmen” covers sundry Germans (including those famous sailors, the Swiss Germans), and that “Dagos” extends to the Romance peoples, if peoples they be. I’m amazed that British Eurosceptics haven’t discovered the expression.